actually you are totally mistaken. the NE64 does not have JTAG at all.
unless you are going to use NE64 as JTAG MASTER in what case
the speed is most likely irrelevant. in any case achiving 25MHz
sustained JTAG update in master mode is almost not possible
without dedicated JTAG master hardware.

but i assume you are talking about the BDM not JTAG?
for BDM yes the 25MHz is important that is dictating the bit timing